Thermosiphon Systems for Electronic Devices

ABSTRACT

A thermosiphon system includes a condenser, an evaporator, and a condensate line fluidically coupling the condenser to the evaporator. The condensate line can be a tube with parallel passages can be used to carry the liquid condensate from the condenser to the evaporator and to carry the vapor from the evaporator to the condenser. The evaporator can be integrated into the tube. The condenser can be constructed with an angled core. The entire assembly can be constructed using a single material, e.g., aluminum, and can be brazed together in a single brazing operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/548,046, filed Jul. 12, 2012, the entire disclosure of which isincorporated by reference.

TECHNICAL FIELD

This invention relates to thermosiphon systems to remove heat fromelectronic devices.

BACKGROUND

Computer users often focus on the speed of computer microprocessors(e.g., megahertz and gigahertz). Many forget that this speed often comeswith a cost—higher power consumption. This power consumption alsogenerates heat. That is because, by simple laws of physics, all thepower has to go somewhere, and that somewhere is, in the end, conversioninto heat. A pair of microprocessors mounted on a single motherboard candraw hundreds of watts or more of power. Multiply that figure by severalthousand (or tens of thousands) to account for the many computers in alarge data center, and one can readily appreciate the amount of heatthat can be generated. The effects of power consumed by the criticalload in the data center are often compounded when one incorporates allof the ancillary equipment required to support the critical load.

Many techniques may be used to cool electronic devices (e.g.,processors, memories, and other heat generating devices) that arelocated on a server rack tray. For instance, forced convection may becreated by providing a cooling airflow over the devices. Fans locatednear the devices, fans located in computer server rooms, and/or fanslocated in ductwork in fluid communication with the air surrounding theelectronic devices, may force the cooling airflow over the traycontaining the devices. In some instances, one or more components ordevices on a server tray may be located in a difficult-to-cool area ofthe tray; for example, an area where forced convection is notparticularly effective or not available.

The consequence of inadequate and/or insufficient cooling may be thefailure of one or more electronic devices on the tray due to atemperature of the device exceeding a maximum rated temperature. Whilecertain redundancies may be built into a computer data center, a serverrack, and even individual trays, the failure of devices due tooverheating can come at a great cost in terms of speed, efficiency, andexpense.

Thermosiphons are heat exchangers that operate using a fluid thatundergoes a phase change. A liquid form of the fluid is vaporized in anevaporator, and heat is carried by the vapor form of the fluid from theevaporator to a condenser. In the condenser, the vapor condenses, andthe liquid form of the fluid is then returned via gravity to theevaporator. Thus, the fluid circulates between the evaporator and thecondenser without need of a mechanical pump.

SUMMARY

As noted above, electronic devices, e.g., computer components, such asprocessors and memories, generate heat. A thermosiphon system can beused to remove heat from such an electronic device. Although somesystems have been proposed for removing heat from computer components,the limited space available in the server rack environment introduces anadditional challenge to thermosiphon system design. In addition, forcommercial applicability, the thermosiphon needs to operate with highefficiency.

Several approaches are described, which can be used individually or incombination in order to improve efficiency. A tube with parallelpassages can be used to carry the liquid condensate from the condenserto the evaporator and to carry the vapor from the evaporator to thecondenser. The evaporator can be integrated into the tube. The condensercan be constructed with an angled core. The entire assembly can beconstructed using a single material, e.g., aluminum, and can be brazedtogether in a single brazing operation. The evaporator loading plate canapply pressure along a central axis.

In one aspect, a thermosiphon system includes a condenser, anevaporator, and a condensate line fluidically coupling the condenser tothe evaporator. The condensate line includes a tube having a centralpassage and a pair of outer passages positioned on opposite lateralsides of the central passage and extending parallel to the centralpassage. The central passage is positioned to carry a vapor phase of theworking fluid from the evaporator to the condenser. The pair of outerpassages are positioned to carry a liquid phase of a working fluid fromthe condenser to the evaporator.

Implementations can include one or more of the following features. Thetube may be a flattened rectangular body, the body having a widthgreater than its height. The outer passages may be positioned adjacentside walls of the rectangular tube. The central passage may extend froma top wall to a bottom wall of the rectangular tube. The tube mayinclude a support strut positioned in the central passage and extendingalong a length of the tube. The pair of outer passages may have across-sectional area about 5-25% of the central passage. The tube may bea unitary brazed body. The evaporator may include an evaporator pan witha surface that is recessed relative to a bottom of the central passage.Each of the pair of outer passages may have an aperture positioned overor adjacent the evaporator pan. The evaporator may include a pluralityof fins projecting upwardly from the evaporator pan.

In another aspect, a thermosiphon system includes a condenser, anevaporator, and a condensate line fluidically coupling the condenser tothe evaporator. The evaporator includes an evaporator pan with a surfacerecessed relative to a floor of the condensate line, and a plurality ofprojections extending upwardly from the evaporator pan with tops of thefins positioned above the floor of the condensate line.

Implementations can include one or more of the following features. Theplurality of projections may include a plurality of fins. The pluralityof fins may be arranged substantially in parallel. The fins haveundulations along their length. The undulations may have a pitch between1 and 2 mm and an amplitude between 0.1 and 0.5 mm. A ceiling of theevaporator may be flush with a top of the central passage. Theevaporator and the condensate line may be a unitary brazed body. Theevaporator pan and the projections may be copper. The condensate linemay be a tube having a central passage and a pair of outer passagespositioned on opposite lateral sides of the central passage andextending parallel to the central passage, the central passage may bepositioned to carry a vapor phase of the working fluid from theevaporator to the condenser, and the pair of outer passages may bepositioned to carry a liquid phase of a working fluid from the condenserto the evaporator. Each of the pair of outer passages may have anaperture positioned over or adjacent the evaporator pan.

In another aspect, a thermosiphon system includes an evaporator, acondenser, and a condensate line. The condenser includes a body having afirst side with an opening, a second side on the opposite side of thebody from the first side, a central channel extending from the openingtoward the second side, and a plurality of parallel chambers extendinglaterally from the central channel. A floor of the central channel andthe plurality of parallel chambers is canted with an end of the centralchannel closer to the second side being higher than an end of thecentral channel at the opening. The condensate line fluidically couplesthe opening of the condenser to the evaporator.

Implementations can include one or more of the following features. Thecondensate line may include a tube having a central passage and a pairof outer passages positioned on opposite lateral sides of the centralpassage and extending parallel to the central passage, the centralpassage may be positioned to carry a vapor phase of the working fluidfrom the evaporator to the condenser, and the pair of outer passages maybe positioned to carry a liquid phase of a working fluid from thecondenser to the evaporator. The body may include a cavity and aplurality of walls that divide the cavity into the plurality of parallelchambers. A plurality of heat conducting fins may project outwardly fromthe body. The plurality of heat conducting fins may project verticallyfrom the body. Tops of the plurality of heat conducting fins may lie ina horizontal plane. The floor of the central channel and the pluralityof parallel chambers may be canted at an angle of 1°-30°, about 7.5°,relative to horizontal.

In another aspect, a method of assembling a thermosiphon system includesproviding a condenser, an evaporator, and a condensate line, andsimultaneously brazing the condenser, the evaporator, and the condensateline to form a unitary body in a single brazing process.

Implementations can include one or more of the following features. Thecondenser, the evaporator, and the condensate line may consist ofaluminum. The condenser and the condensate line may consist of aluminum,and the evaporator may include a cooper evaporator pan. The singlebrazing process may be heating the condenser, the evaporator, and thecondensate line simultaneously to a temperature of between about 580 and620° C.

One or more of the following advantages may be realized. Thethermosiphon system can fit within the limited horizontal and verticalspace of the server rack. A thin layer of liquid can be maintained inthe evaporator over the region where the evaporator contacts theelectronic device, thus reducing thermal resistance of the evaporator toabsorption of heat from the electronic device. Fins in the evaporatorthat project above the fluid level can improve heat transfer and/orreduce sensitivity to the angle of installation of the thermosiphonsystem. The angled core can enhance thermal performance by, among otherthings, exposing more of the condensate fins to vapor. The generallyrectangular extruded tube can reduce the part count as compared toprevious systems, thus reducing manufacturing cost and increasing yield.The single-material construction can also reduce manufacturingcomplexity, e.g., by allowing the entire system to be brazed as a singleunit, which can decrease the likelihood of leaks. The generallyrectangular extruded tube can provide a superior form factor, e.g.,similar functionality with less space occupied by extraneous material.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects,features, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a side view of a server rack and a server-racksub-assembly configured to mount within the rack.

FIGS. 2 and 3 illustrate a side view and a top view of a server racksub-assembly.

FIG. 4 illustrates a perspective view of a server rack sub-assembly (butomits the printed circuit board and heat generating elements to providea view of more of the frame).

FIG. 5 illustrates a perspective view of a thermosiphon system.

FIG. 6 illustrates a cross-sectional view of a condensate/vapor line,which can be a view taken along line 6-6 of FIG. 5.

FIG. 7 is a perspective view of a mounting bracket.

FIG. 8 illustrates a cross-sectional frontal view of an implementationof the evaporator.

FIGS. 9A and 9B illustrate cross-sectional side views of animplementation of an evaporator from a thermosiphon system, which can bea view taken along lines 9A-9A and 9B-9B of FIG. 8, respectively.

FIG. 9C illustrates an isometric cut-away view of the implementation ofthe evaporator of FIGS. 9A and 9B.

FIG. 9D illustrates a top view of the implementation of the evaporatorof FIGS. 9A and 9B.

FIG. 9E illustrates a detailed view of an implementation of anevaporator fins in the implementation of the evaporator of FIGS. 6A and6B.

FIGS. 10 and 11 illustrate side views, cross-sectional, of a condenserfrom the thermosiphon system.

FIGS. 12 and 13 illustrate top views, cross-sectional, of thethermosiphon system of FIGS. 8 and 9.

FIG. 14 illustrates a perspective view, cut away, of a condenser fromthe thermosiphon system.

FIG. 15 illustrates an exploded perspective view of a thermosiphonsystem.

FIGS. 16 and 17 illustrate a top view and a side view, cross-sectional,of another implementation of a condenser.

FIG. 18 illustrates a perspective view, cut away, of the othercondenser.

FIG. 19 is an expanded top view, cross-sectional, of a chamber in thecondenser.

FIG. 20 illustrates a top view, cross-sectional, of anotherimplementation of a condenser.

FIG. 21 illustrates a side view, cross-sectional, of the implementationof the condenser in FIG. 20.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This document discusses a thermosiphon system that can be implemented toremove heat from an electronic device, e.g., a component of computingequipment, such as a processor or memory. The evaporator of thethermosiphon system contacts the electronic device so that theelectronic device experiences a conductive heat transfer effect. Thus,the thermosiphon system can act as a heat sink for the electronicdevice, reducing the likelihood of overheating and subsequent failure ofthe electronic device.

In particular, the thermosiphon system can be mounted on or integratedwith a server rack sub-assembly for insertion into a server rack. Theserver rack sub-assembly can contain or support a number ofheat-generating electronic devices, and the evaporator of thethermosiphon system can contact one or more of the electronic devices.In addition, the thermosiphon system can be mounted on a circuit cardassembly, a daughter card, and/or other boards that carryheat-generating electronic devices.

FIG. 1 illustrates an example system 100 that includes a server rack105, e.g., a 13 inch or 19 inch server rack, and multiple server racksub-assemblies 110 mounted within the rack 105. Although a single serverrack 105 is illustrated, server rack 105 may be one of a number ofserver racks within the system 100, which may include a server farm or aco-location facility that contains various rack mounted computersystems. Also, although multiple server rack sub-assemblies 110 areillustrated as mounted within the rack 105, there might be only a singleserver rack sub-assembly. Generally, the server rack 105 definesmultiple slots 107 that are arranged in an orderly and repeating fashionwithin the server rack 105, and each slot 107 is a space in the rackinto which a corresponding server rack sub-assembly 110 can be placedand removed. For example, the server rack sub-assembly can be supportedon rails 112 that project from opposite sides of the rack 105, and whichcan define the position of the slots 107. The slots, and the server racksub-assemblies 110, can be oriented with the illustrated horizontalarrangement (with respect to gravity). Alternatively, the slots 107, andthe server rack sub-assemblies 110, can be oriented vertically (withrespect to gravity), although this would require some reconfiguration ofthe evaporator and condenser structures described below. Where the slotsare oriented horizontally, they may be stacked vertically in the rack105, and where the slots are oriented vertically, they may be stackedhorizontally in the rack 105.

Server rack 105, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 105 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 105 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 110 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 110 may be a “tray” ortray assembly that can be slidably inserted into the server rack 105.The term “tray” is not limited to any particular arrangement, butinstead applies to motherboard or other relatively flat structuresappurtenant to a motherboard for supporting the motherboard in positionin a rack structure. In some implementations, the server racksub-assembly 110 may be a server chassis, or server container (e.g.,server box). In some implementations, the server rack sub-assembly 110may be a hard drive cage.

Referring to FIGS. 2, 3 and 4, the server rack sub-assembly 110 includesa frame or cage 120, a printed circuit board 122, e.g., a motherboard,supported on the frame 120, one or more heat-generating electronicdevices 124, e.g., a processor or memory, mounted on the printed circuitboard 122, and a thermosiphon system 130. One or more fans 126 can alsobe mounted on the frame 120.

The frame 120 can include or simply be a flat structure onto which themotherboard 122 can be placed and mounted, so that the frame 120 can begrasped by technicians for moving the motherboard into place and holdingit in position within the rack 105. For example, the server racksub-assembly 110 may be mounted horizontally in the server rack 105 suchas by sliding the frame 120 into the slot 107 and over a pair of railsin the rack 105 on opposed sides of the server rack sub-assembly110—much like sliding a lunch tray into a cafeteria rack. Although FIGS.2 and 3 illustrate the frame 120 extending below the motherboard 122,the frame can have other forms (e.g., by implementing it as a peripheralframe around the motherboard) or may be eliminated so that themotherboard itself is located in, e.g., slidably engages, the rack 105.In addition, although FIG. 2 illustrates the frame 120 as a flat plate,the frame 120 can include one or more side walls 121 (see FIG. 4) thatproject upwardly from the edges of the flat plate, and the flat platecould be the floor of a closed-top or open-top box or cage.

The illustrated server rack sub-assembly 110 includes a printed circuitboard 122, e.g., a motherboard, on which a variety of components aremounted, including heat-generating electronic devices 124. Although onemotherboard 122 is illustrated as mounted on the frame 120, multiplemotherboards may be mounted on the frame 120, depending on the needs ofthe particular application. In some implementations, the one or morefans 126 can be placed on the frame 120 so that air enters at the frontedge (at the left hand side in FIG. 3) of the server rack sub-assembly110, closer to the front of the rack 105 when the sub-assembly 110 isinstalled in the rack 105, flows (see arrow A in FIG. 4) over themotherboard and over some of the heat generating components on themotherboard 122, and is exhausted from the server rack assembly 110 atthe back edge (at the right hand side in FIG. 3), closer to the back ofthe rack 105 when the sub-assembly 110 is installed in the rack 105. Theone or more fans 126 can be secured to the frame 120 by brackets 127.Thus, the fans 126 can pull air from within the frame 120 area and pushthe air after it has been warmed out the rack 105. An underside of themotherboard 122 can be separated from the frame 120 by a gap.

As shown in FIGS. 2-5, the thermosiphon system 130 includes anevaporator 132, a condenser 134, and a condensate/vapor line 136connecting the evaporator region 132 to the condenser 134. Thecondensate/vapor line 136 includes at least two parallel passages, e.g.,three parallel passages.

In the implementation shown in FIG. 6, the condensate/vapor line 136includes a central passage 138 a and two outer passages 138 b. The twoouter passages 138 b are positioned on opposite lateral sides of thecentral passage 138 a and extend parallel to the central passage 138 a.In particular, the outer passages 138 b can be positioned adjacent tothe side walls 136 a of the tube that provides the condensate/vapor line136. In addition, the outer passages 138 b can be positioned adjacentthe bottom floor of the tube that provides the condensate/vapor line136. In operation, the central passage 138 a carries a vapor phase ofthe working fluid from the evaporator 132 to the condenser 130, and thepair of outer passages 138 b carry a liquid phase of the working fluidfrom the condenser 130 to the evaporator 132.

The condensate/vapor line 136 can be constructed as a flattenedrectangular body, having a width W (measured perpendicular to the longaxis of the evaporator) greater than its height H (measuredperpendicular to the surface of the printed circuit board). As shown inFIG. 6, the evaporator 132 and the condensate/vapor line 136 can includea plurality of partitions, including two outer partitions 136 b. Thevolume between each outer partition 136 b and the side wall 136 a andbottom wall 137 b defines an outer passage 138 b. The volume between theouter partitions 136 b, between the top wall 137 a and the bottom wall137 b, can define the central passage 138 a.

In some implementations, the plurality of partitions also includes acentral partition 135. The central partition 135 can extend from the topwall 137 a to the bottom wall 137 b of the condensate/vapor line 136. Asshown in FIG. 6, the central partition 135 divides the central passage138 a into two or more passages. The central partition 135 can be asupport strut that extends from the top wall 137 a to the bottom wall137 b to provide improved structural strength and stability.

The pair of outer passages 138 a, taken together, can be about 5-25% ofthe cross sectional area of the central passage 138 b, e.g., each outerpassage 138 a be about one-third the width of the central passage 138 b.The outer partitions 136 a can be located on opposite lateral sides ofthe central partition 136 b, and run parallel to the central partition136 b.

Returning to FIGS. 2, 5 and 6, the evaporator 132 contacts theelectronic device 124 so that heat is drawn by conductive heat transferfrom the electronic device 124 to the evaporator 132. In particular, thebottom of the evaporator 132 contacts the top of the electronic device124. In operation, heat from the electronic device 124 causes a workingfluid in the evaporator 132 to evaporate. The vapor then passes throughcondensate/vapor line 136, particularly through the central passage 138b, to the condenser 134. Heat is radiated away from the condenser 134,e.g., into air blown or drawn by the one or more fans 126 that passacross the condenser 134, causing the working fluid to condense. Thecondensed working fluid can flow back through the condensate/vapor line136, particularly through the outer passages 138 a, to the evaporator132.

The evaporator 132 can be put in thermal contact with the electronicdevice 124 by a mounting bracket 150 that applies urges the evaporator132 towards the electronic device 124. The mounting bracket 150 can beattached to the printed circuit board 122 by fasteners 152.

Referring to FIGS. 2, 5, and 7, the mounting bracket, as mounted on thecircuit board 122, can have a downwardly extending projection 154, e.g.,a convex bump that applies a greater amount of pressure to the center ofthe evaporator 132. In some implementations, the projection 154 can beelongated, e.g., along the longitudinal axis of the condensate/vaporline 136. The projection 154 can span a length (measured parallel to thepassages of the condensate/vapor line 136) of the evaporator 132.Additionally, the projection 154 can be centered across a width(measured perpendicular to the passages of the condensate/vapor line136) of the evaporator 132. This configuration applies a greater amountof pressure along the center line of the evaporator, and can prevent thebottom of the evaporator 132 from bending away from the top of theelectronic device 124. Maintaining good contact between the bottom ofthe evaporator 132 and the top of the electronic device 124 can improvecooling efficiency.

As shown in the implementation of FIGS. 5 and 7, the mounting bracket150 includes a horizontally extending crosspiece 155. The downwardlyextending projection 154 is formed on the bottom surface of thecrosspiece 155. The crosspiece 155 extends horizontally between two sideflanges 156 extend downwardly from the cross-piece 155. When installedon the circuit board 122, the side flanges 156 can sit on either side ofthe electronic device 124. Springs 157, which can bear against sideflanges 56, urge the mounting bracket 150 downwardly.

As shown in FIGS. 2 and 3, the condenser 134 can be located on anopposite side of one or more of the one or more fans 126 from theevaporator 132. Alternatively or in addition, the condenser 134 can belocated on a same side of one or more of the one or more fans 126 as theevaporator 132. The condensate/vapor line 136 can extend below the fans126.

As shown in FIG. 2, a portion 133 of the condensate/vapor line 136 canbe at a slight (non-zero) angle so that gravity causes the condensedworking fluid to flow back through the condensate/vapor line 136 to theevaporator 132. The angle can be 1°-30°, e.g., 7.5°, relative to therelative to horizontal. Since the frame 120 is typically heldhorizontally in operation, the angle can be 1°-30° relative to thesurface of the frame. Thus, in some implementations, at least a portion133 of the condensate/vapor line 136 is not parallel to the main surfaceof the frame 120. For example, the condenser-side end of thecondensate/vapor line 136 can be about 1-5 mm, e.g., 2 mm, above theevaporator-side end of the condensate/vapor line 136. However, it isalso possible for the condensate/vapor line 136 to be horizontal, oreven at a slightly negative angle (although the positive angle providesan advantage of gravity improving flow of the liquid from the condenserto the evaporator). Because there can be multiple heat generatingelectronic devices on a single motherboard, there can be multipleevaporators on the motherboard, where each evaporator corresponds to asingle electronic device. As shown in FIGS. 2 and 3, there is a firstevaporator 132 and a second evaporator 132 as well as a first electronicdevice 124 and a second electronic device 124. The condensate/vapor line136 connecting the first evaporator to the second evaporator can belevel, or can have a slight positive angle (with the evaporator 132 andend of the tube 136 that is farther from the condenser 130 being lowerthan the other evaporator).

During operation, the top surface of the liquid inside the condenserwill be above the top surface liquid height in the evaporator, e.g., by1 to 10 mm. It can be easier to achieve this with a condensate/vaporline 136 that is at a slight (positive non-zero) angle, but properselection of the thermal and mechanical properties of the working fluidin view of the expected heat transport requirements for the thermosiphonsystem may still achieve this for a condensate/vapor line 136 that ishorizontal or at a slightly negative angle.

During operation, the liquid phase of a working fluid can fill at leasta bottom portion of an interior volume of the outer passages 138 a ofthe condensate/vapor line 136, with the bottom portion extending fromthe condenser to the evaporator region, and a vapor phase of the workingfluid can pass through the central passage 138 b of the condensate/vaporline 136. Furthermore, the liquid phase of the working fluid can flowfrom the outer passages 138 a into the central passage 138 b through oneor more apertures 139. The liquid phase of the working fluid can fill atleast a bottom portion of an interior volume of the condenser 124. Someportion of the outer passages 138 a can carry vapor. Due to theseparation of the central passage 138 b from the outer passages 138 a bythe walls of the outer partitions 136 a, shear stress between the liquidand the vapor phases of the working fluid flowing in opposite directionscan be reduced, thus improving condensate flow to the evaporator andimproving efficiency.

In some implementations, the condenser 134 can be located at a heightabove the evaporator 132 such that a liquid phase of the working fluidfills a portion of an interior volume of the condensate/vapor line 136,i.e., the outer passages 138 a, and such that during operation a topsurface of the liquid phase has at a non-zero angle relative tohorizontal from the condenser 132 to the evaporator 134, and a vaporphase of the working fluid can pass through a portion of the interiorvolume of the condensate/vapor line 136, i.e., the central passage 138b, the portion extending from the condenser 132 to the evaporator 134.

FIGS. 2-4 illustrate a thermosiphon system 130 with multiple evaporators132; each evaporator 132 can contact a different electronic device 124,or multiple evaporators 132 could contact the same electronic device,e.g., if the electronic device is particularly large or has multipleheat generating regions. As shown in FIGS. 2-4, the multiple evaporators132 can be connected by the condensate/vapor lines 136 to the condenser134 in series, i.e., a first condensate/vapor line connects thecondenser to a first evaporator, and a second condensate/vapor lineconnects the first evaporator to a second evaporator. Alternatively,some or all of the multiple evaporators 132 can be connected by thecondensate/vapor lines 136 to the condenser 134 in parallel, i.e., afirst condensate/vapor line connects the condenser to a firstevaporator, and a second condensate/vapor line connects the condenser134 to a second evaporator. An advantage of a serial implementation isfewer tubes, whereas an advantage of parallel tubes is that the tubewidths can be smaller.

FIGS. 2-4 and 6 illustrate a thermosiphon system 130 in which aflattened rectangular body having a central passage and a pair of outerpassages positioned on opposite lateral sides of the central passage isused for the condensate flow from the condenser 134 to the evaporator132 and for vapor flow from the evaporator 132 to the condenser 134.Thus, in this implementation the fluidical coupling between theevaporator 132 and the condenser 134 consists of the combined condensateand vapor transfer line. A potential advantage of the combinedcondensate and vapor transfer line is that the line can be connected toa side of the condenser, reducing the vertical height of the systemrelative to a system with a separate line for the vapor, since the vaporline is typically coupled to or near the top of the evaporator. Thecondensate/vapor line 136 can be a tube or pipe, e.g., of copper oraluminum.

Furthermore, the condensate/vapor line 136 can be manufactured by ametal extrusion process. The complete assembly of condensate/vapor line136 and condenser 124 can have a limited number of parts, e.g., thevapor line 135 is constructed as a flattened rectangular body, which canreduce the part count as compared to a similar system wherein thecondensate/vapor line is not built as a flattened rectangular body. Thereduction in part count can result in lower manufacturing complexity,lower manufacturing cost, and higher manufacturing yield.

FIGS. 8 and 9A-9E illustrate a thermosiphon system 130 in which theevaporator 132 includes a chamber 146 and a plurality of evaporator fins142. The housing can include a base 140 and a case 144 that is securedto the base 140. The case 144 can be provided by the tube of thecondensate/vapor line 136. An opening 145 can be formed in the bottomsurface of the condensate/vapor line 136. The base 140 abuts thecondensate/vapor line 136 and surrounds the opening 145. The opening 145may be of the same shape as the base, e.g., square. The volume sealedabove the base 140 inside the case 144 provides the chamber 146 for thecondenser 132.

The top surface of the base 140 provides an evaporator pan 143. That is,the top surface 140 includes a portion i) that is recessed relative tothe bottom of the central passage 138 b, and ii) in which the liquidphase of the working fluid 160 collects. For example, the top surface ofthe evaporator pan can be recessed relative to the bottom of the centralpassage 138 b by about 1 mm to 5 mm, e.g., 2 mm.

The evaporator fins 142 project upwardly from the evaporator plan 143 sothat they are above the bottom of the central passage 138 b. When theliquid phase of the working fluid overflows the evaporator pan 143, itfloods the bottom section of the inner passage 138 b. Thus, the bottomof the inner passage 138 b can be considered a floodplain. In addition,this ensures that the evaporator fins 142 remain only partiallysubmerged in the liquid phase of the working fluid.

The housing can be a flattened rectangular body, of the same outsidedimensions as the outside dimensions of the condensate/vapor line 136.The housing can also comprise extensions of the outer passages 138 a andthe central passage 138 b of the condensate/vapor line 136.

The base 140 can be formed of the same material as the housing, e.g.,aluminum. Alternatively, the base 140 can be formed of a differentthermally conductive material, e.g., copper. The housing, e.g., thebottom of the base 140, can directly contact the electronic device 124,e.g., the top surface of the electronic device 124. Alternatively, thehousing, e.g., the bottom of the base 140, can be connected to theelectronic device 124, e.g., the top surface of the electronic device124, by a thermally conductive interface material 141, e.g., a thermallyconductive pad or layer, e.g., a thermally conductive grease oradhesive.

The evaporator fins 142 contact the bottom interior surface of thehousing, e.g., the top surface of the base 140. The evaporator finsproject upwardly from the evaporator pan 143. Thus, the evaporator fins142 provide a thermally conductive area that transfers heat from thebase 140 to the working fluid 160. The tops of the fins 142 can projectabove the floor of the condensate/vapor line 136.

The fins can be arranged substantially in parallel. In someimplementations, the fins extend generally parallel to the width of thecentral passage 136 b, i.e., perpendicular to the length of thecondensate/vapor line 136.

In addition, the evaporator fins 142 can be configured to draw theworking fluid 160 away from the base 140 by capillary action. Forexample, the evaporator fins 142 can be stamped or otherwise imprintedwith features, e.g., grooving, which tends to draw the working fluidupward.

In some implementations, the fins can have undulations along theirlength. The undulations can have a pitch between 1 mm and 2 mm and anamplitude between 0.1 and 0.5 mm. As shown in in FIG. 9E, theseundulations can cause some of the liquid phase of the working fluid 160to move up the fins 142 by capillary action. This can improve theefficiency of the evaporator 132 by exposing more of the surface area ofthe fins 142 to the liquid phase of the working fluid.

The fins can be constructed of the same material as the evaporator,e.g., aluminum. Alternatively, the fins can be constructed of adifferent thermally conductive material, e.g., copper.

The chamber 146 can comprise extensions of the outer passages 138 a andthe central passage 138 b. The top of the chamber 146 can be flush withthe top of the central passage 138 b. A plurality of apertures 139 areformed in the outer passages 138 a in a region above the fins 142.Although FIGS. 9A and 9D illustrate two apertures 139, there could bemore than two apertures.

In operation, the working fluid 160, in liquid form, flows from theouter passages 138 a into the central passage 138 b and the evaporatorpan 143. The working fluid 160 can flow through the apertures 139 andonto the evaporator fins 142 (see FIGS. 9A, 9D and 9E). The workingfluid can fill a volume between the base 140 and the bottom of the outerpassages 138 a, thereby forming a thin layer 162 of working fluid on theevaporator fins 142. In particular, the thin layer 162 can be formed inthe valleys between the undulations of the fins. A remainder of theworking fluid can flow down the outer passages 138 a to anotherevaporator 132. By creating a thin layer 162 of the working fluid 160,the thermal resistance of the evaporator is effectively reduced (becausethe working fluid can evaporate more readily from a thin layer,permitting greater heat transfer).

Turning now to the condenser 132, the condenser 132 includes a pluralityof chambers, and a plurality of heat conducting fins. The chambers canbe parallel and vertically-extending. The top ends of the chambers canbe closed off, i.e., there is no top header that interconnects the topends of the chambers.

FIGS. 10-14 illustrate a first implementation of the condenser 134 thathas a body 170 having cavity 174 formed therein, and a plurality ofwalls 172 in the cavity that divide the cavity 174 into a plurality ofparallel vertically-extending chambers 174 a. The chambers 174 a can beparallel and vertically-extending. The top ends of the chambers 174 acan be closed off, i.e., there is no top header that interconnects thetop ends of the chambers 174 a. The walls 172 act as a condensationsurface and to conduct heat from the vapor, through the body to thefins.

The cavity 174 also includes a central channel 176 with an opening tothe exterior of the body 170 which is coupled to the condensate line136. The vertically-extending chambers 174 a can extend laterally fromthe central channel 176, and the chambers 174 a can extend parallel tothe long axis of the body 170 (i.e., the body has a length greater thanits width, and the long axis is along the length). The central channel176 can extend laterally perpendicular to the long axis. When thecondenser 134 is installed on the frame, the central channel 176 can runfrom the front toward the rear of the body 170. A first set of thevertically-extending chambers 174 can extend laterally from a first sideof the central channel 176, and a second set of the plurality ofvertically-extending chambers 174 can extend laterally from an oppositesecond side of the central channel 176. The body 170 can be a generallyrectangular solid, although other shapes are possible.

This implementation of the condenser 134 that has a plurality of heatconducting fins 180 that project outwardly from the body 170. Forexample, the fins 180 can project vertically from the body 170. The fins170 can be generally flat, narrow sheets. The fins 180 can project inparallel to each other from the body 170, and can be spaced apart with aregular pitch along a direction normal to their flat primary surfaces.In some implementations, the fins 180 include at least a first pluralityof fins 180 a that project upwardly from the top surface of the body170. In some implementations, the fins 180 also include a secondplurality of fins 180 b that project downwardly from the bottom surfaceof the body 170.

When the condenser 134 is installed on the frame, the fins 180 can beoriented with their length extending parallel or generally parallel tothe direction of air flow generated by the fans, e.g., with their lengthrunning from the front toward the rear of the of the body 170. The fins180 can be oriented with their long axis perpendicular to, or at aslight angle to, the long-axis of the chambers 174 a and/or the body170.

Returning to FIG. 2, the condenser 134 can rest on the frame 120, andthe fins 180 b that project downwardly from the bottom surface of thebody 170 can project below the plane of the motherboard 122. This canimprove the available surface area for the fins to improve radiatingefficiency of the condenser 134. This can also assist in limiting thevertical height of the condenser 134 so that the thermosiphon system 130is compatible with the limited height available in the server rackenvironment. For example, a total height from a bottom of the tray to atop of the heat conducting fins can be at most 6 inches, e.g., at most 4inches.

As shown in the implementation of FIGS. 10 and 11, the outer passages138 a can connect to the cavity 174 of the condenser 134 at positionsvertically lower than the central passage 138 b. The bottom of thecentral passage 138 b can be vertically level with the top of the outerpassages 138 a.

Referring to FIG. 10, the floor 175 of the cavity 174 can be sloped,with the side 175 a abutting the condensate/vapor line 136 at avertically lower position than the side 175 b on the opposite side ofthe cavity 174 from the condensate/vapor line 136. The floor 175 of thecavity 174 can be sloped at an angle of 1°-30°, e.g., 7.5°, relative tohorizontal. Since in operation the fins 180 typically projectvertically, the floor 175 of the cavity 174 can be at angle of 60°-89°relative to fins 180. In some implementations, the floor 175 of thecavity 174 is sloped at the same angle as the portion 133 of thecondensate/vapor line 136.

The fluid level and the vertical offset of the central passage 138 b canbe set such that the openings to the outer passages 138 a are at leastpartially covered with liquid, and the opening to the central passage138 b is exposed only to vapor. The sloped floor 175 of the cavity 174can cause the liquid phase of the working fluid to pool in the cavity174 near the condensate/vapor line 136, which improves the likelihoodthat the entrances to the outer passages 138 a remain covered by theliquid phase of the working fluid. Furthermore, the sloped floor 175 ofthe cavity 174 can increase the proportion of the vapor phase of theworking fluid in the portion of the cavity 174 on the opposite end fromthe condensate/vapor line 136, thus keeping more of the fins 180 in thatregion exposed to the vapor phase of the working fluid.

Referring to FIG. 15, the evaporator 132, the condenser 134, and thecondensate/vapor line 136 can be constructed of the same material, e.g.,aluminum. Constructing the evaporator 132, the condenser 134, and thecondensate/vapor line 136 using the same material can reducemanufacturing complexity. For example, the evaporator 132, the condenser134, and the condensate/vapor line 136 can be formed in a single brazingprocess, wherein all three parts are simultaneously heated to atemperature sufficient to braze the parts together. For example, foraluminum, all the parts can be heated to a temperature between about580-620° C. This provides a unitary part that is less likely to developleaks. In addition, reducing the number of brazing steps can reducemanufacturing cost.

Alternatively, a portion of the evaporator 132, e.g., a bottom floor ofthe evaporator which contacts the heat-generating electronic device 124,may be constructed of a different material, e.g., copper. Thisconfiguration can also reduce manufacturing complexity to some degree,as the condenser 134 and the condensate/vapor line 136 can still beformed together in a single brazing process.

FIGS. 16-18, illustrate a second implementation of the condenser 134that also has a plurality of heat conducting fins 180 that projectoutwardly from the body 170. However, in this implementation, thevertically-extending chambers 174 a extend vertically from the centralchannel 176. In particular, the body can include a bottom header 190which contains the central channel 176, and plurality of tubes 192 thatproject vertically from the bottom header 190 and contain thevertically-extending chambers 174 a. The condensate line 136 isfluidically coupled to the bottom header 190 of the condenser 134.

Each chamber 174 a can be formed by its own, and the walls 172 that formthe boundaries of vertically extending chamber 174 a can be walls of thetubes 192. The chambers 174 a can extend perpendicular to the long axisof the body 170. Although the vertically extending chambers 174 a areconnected to a bottom header 190, the top ends of the chambers 174 a canbe closed off, i.e., the condenser 134 does not include a top header.

The fins 180 can project horizontally from the body 170, e.g.,horizontally from the tubes 192. The fins 180 can extend parallel to thelong axis of the bottom header 190. The fins 180 can be generally flat,narrow sheets. The fins 180 can project in parallel to each other fromthe body 170, and can be spaced apart, e.g., vertically spaced apart,with a regular pitch along a direction normal to their flat primarysurfaces.

When the condenser 134 is installed on the frame, the fins 180 can beoriented with their length extending parallel or generally parallel tothe direction of air flow generated by the fans, e.g., with their lengthrunning from the front toward the rear of the of the body 170. The fins180 can be oriented with their long axis parallel to the long-axis ofthe chambers 174 a.

In either implementation of the condenser, both the body 170 of thecondenser 134 and the fins 180 can be formed of a material with a goodterminal conductivity, comparable or better than aluminum, e.g., of atleast 200 W/mK. A nickel plating can be used to solder the fins 180 tothe body 170, or the fins 180 can be brazed to the body 170.

Referring to FIGS. 20-21, in another implementation of the condenser,the ends of the chambers 174 a further from the central channel 176 canbe connected by a channel 178 a. The channel 178 a is fluidicallyconnected by a channel 178 b to an end of the central channel 176further from the outer tube 138 a. The channels 178 a and 178 b can beshorter than the chambers 174 a, e.g., the channel 178 a can beconnected to the bottom of chambers 174 a. Optionally, an additionallaterally-extending chamber 178 c can be positioned over the channel 178b to provide additional surface area for condensing. The condenser canotherwise be constructed similarly to the implementation shown in FIGS.10-14. A potential advantage of this configuration is that at high flowcapacity, fluid that would otherwise build up at the end of the chamber174 a farther from the central channel 176 and be unable to flow backdue to vapor flow, can instead flow through the channels 178 a and 178 band thus return to the outgoing outer passages 138 a.

Referring to FIGS. 13, 16, 19 and 20, at least some interior surfaces ofthe condenser, e.g., surfaces that bound the cavity 174, can optionallybe texturized. The texturization can apply to either implementation ofthe condenser. The cavity 174 provides an interior volume bounded by asubstantially vertical interior surface, e.g., a surface of one of thewalls 172. The texturization of the interior surface can includeundulations projecting inwardly into the interior volume. Theundulations can be uniform along a vertical first axis, and can projectinto the interior volume along a second axis perpendicular to thevertical first axis. Peaks of the undulations can be spaced apart, e.g.,with a regular pitch, along a third axis perpendicular to the first axisand the second axis. The third axis can be parallel to the long axis ofthe body 170 and/or the chamber 174 a. Each chamber 174 a can have alength along the third axis and a width along the second axis with thelength being greater than the width. The undulations can be smooth,e.g., no discontinuities in the surface along the second axis.

The undulations can have a pitch along the third axis between 0.1 and 1mm and can have an amplitude along the second axis between 0.1 and 1 mm.In some implementations, a ratio of the pitch to the amplitude isbetween about 1:1 to 2:1. In some implementations, the undulations canform a sinusoidal wave. In some implementations, the undulations areformed by a plurality of curved segments in which dK/dS is equal to aconstant value, where K is an inverse of the radius of curvature of theundulation and S is a distance along a curved segment. Other shapes forthe undulations are possible. These undulations can cause thinning ofthe film of condensed working fluid that forms on the vertical interiorsurface, thereby reducing the thermal resistance of the condenser.

The working fluid can be a dielectric, non-flammable fluid with lowtoxicity, although but hydrocarbons such as methanol, ethanol or acetonecan also be suitable. The composition of the working fluid and internalpressure of the thermosiphon system can be selected to provide a boilingpoint of the working fluid in the evaporator at about the desiredoperating temperature for the electronic devices, e.g., around 30-100°C., e.g., 45-55° C. Examples of the working fluid include Vextral XFsold by DuPont, Fluorinert Electronic Liquid FC-72, sold by 3M, andNovec 7100, sold by 3M.

The entire interior of the thermosiphon system 130, including theinterior of the evaporator 132, condenser 134 and vapor/condensate line136, are vacuum filled and sealed. Initial vacuum can be pulled toachieve an internal absolute pressure below 0.05 millibar (5 Pa) toremove air from the thermosiphon system 130, and then the working fluidcan be introduced into thermosiphon system 130.

Although a server rack sub-assembly has been described above, thethermosiphon system could be used with heat-generating electronicdevices mounted on a motherboard that is not part of a server racksub-assembly, e.g., on a motherboard in a desktop computer, or could beused with heat-generating electronic devices that are not mounted on amotherboard. In some implementations, the evaporator fins could bereplaced by a porous wicking material.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. (canceled)
 2. A method of assembling a thermosiphon system,comprising: providing a condenser, an evaporator, and a condensate line;and simultaneously brazing the condenser, the evaporator, and thecondensate line to form a unitary body in a single brazing process. 3.The method of claim 2, wherein the condenser, the evaporator, and thecondensate line comprise aluminum.
 4. The method of claim 2, wherein thecondenser and the condensate line comprise aluminum, and the evaporatorcomprises a copper evaporator pan.
 5. The method of claim 2, wherein thesingle brazing process comprises heating the condenser, the evaporator,and the condensate line simultaneously to a temperature of between about580 and 620° C.
 6. The method of claim 2, wherein the condensercomprises a body having a first side with an opening, a second side onthe opposite side of the body from the first side, a cavity, and aplurality of walls that divide the cavity into a central channel and aplurality of parallel chambers.
 7. The method of claim 6, wherein thecentral channel extends from the opening toward the second side, theplurality of parallel chambers extend laterally from the centralchannel, a plurality of heat conducting fins projecting outwardly andvertically from the body, and the walls extend upwardly from a floor ofthe cavity.
 8. The method of claim 6, wherein a floor of the cavity iscanted such that bottom ends of the walls are vertically staggered, anda floor of the central channel and the plurality of parallel chambersare canted from an end of the central channel closer to the second sideto an end of the central channel at the opening, the opening in thefirst side positioned vertically higher than a location at which atleast one of the walls is connected to the floor.
 9. The method of claim6, wherein the condensate line comprises a tube with an open end that iscoupled, after the simultaneous brazing, to the opening of the firstside such that the tube is vertically spaced above the floor of thecentral channel by a portion of a side wall of the central channel. 10.The method of claim 9, wherein the tube comprises a lower portion tocarry the liquid phase of the working fluid and an upper portion tocarry a vapor phase of the working fluid.
 11. The method of claim 10,wherein the lower portion and the upper portion are both connected,after the simultaneous brazing, to the evaporator at the opening of thefirst side with the upper portion adjacent to and vertically above thelower portion.
 12. The method of claim 6, wherein the plurality ofparallel chambers formed by the plurality of walls are fluidly coupledat open ends that are open and adjacent to the central channel and arefluidly decoupled at closed ends that are opposite the open ends andclosed by sidewalls of the body.
 13. The method of claim 10, wherein theupper portion of the tube comprises a central passage and the lowerportion of the tube comprises a pair of outer passages fluidly coupledto, and positioned on opposite lateral sides of, the central passage andextending parallel to the central passage.
 14. The method of claim 13,wherein the central passage is fluidly coupled, through the opening ofthe first side, to a portion of the cavity that encloses a vapor phaseof the working fluid, and the pair of outer passages are fluidlycoupled, through the opening of the first side, to another portion ofthe cavity that encloses the liquid phase of the working fluid.
 15. Themethod of claim 2, wherein the condenser, the evaporator, and thecondensate line comprise copper.
 16. The method of claim 2, wherein aninterior surface of the condenser is texturized.
 17. The method of claim6, wherein tops of the plurality of heat conducting fins lie in ahorizontal plane and the floor of the central channel and the pluralityof parallel chambers is canted relative to the horizontal plane.
 18. Themethod of claim 17, wherein at least a portion of the condensate lineadjacent the condenser is canted relative to the horizontal plane. 19.The method of claim 17, wherein the portion of the condensate line iscanted at the same angle as the floor of the central channel.
 20. Themethod of claim 6, wherein the floor of the central channel and theplurality of parallel chambers is canted at an angle of 1°-30° relativeto horizontal.
 21. The method of claim 20, wherein the angle is about7.5°.